In the production of carpet tiles, it is common practice to apply a liquid dye solution to the surface pile of the carpet tiles by a printing process. In general, the application ratio is in the range of from 100 percent to 400 percent of the surface pile weight. That is, the dye applied is usually at least equal in weight to the weight of the pile of the carpet, and may be up to four times that weight. Normally the pile is about 4 mm long, and the required penetration of the dye is through about the top two-thirds of the pile. Only in the least densely packed pile is it necessary for the dye to penetrate down to the backing of the carpet (that is the situation where a dye quantity of about four times the weight of the pile has to be applied to it).
In modern production lines, a jet printer applies the dye solution to the carpet tile by means of controlled jets of liquid.
After the printing of the carpet tile, it is usual practice to fix the dyestuff to the surface pile fibres by a heating process. Until now, the heating has been effected using a steaming chamber, in which the carpet tile is raised to a temperature which is in the range of from 90.degree. C. to 100.degree. C. This process raises the temperature of the surface pile, and of the dyestuff contained therein, by the condensation of steam on the surface of the dye and the pile tufts. As a consequence of the condensation, the thermal energy of the latent heat of vaporisation is transferred to those surfaces. This process has the advantage of maintaining the high humidity environment necessary for dye fixation, but it also has a serious drawback. The dye fixation takes a relatively long time, due to the slow rate of conduction of heat through the liquid contained in the surface pile, water being a poor thermal conductor.
Another disadvantage of this steam chamber fixing process is that it is difficult to achieve a uniform temperature at all places in the steaming chamber. Non-uniformity of temperature within the chamber is known to result in a varying percentage of the dyestuff being fixed to the tile, which causes different colour yields between carpet tiles and sometimes an uneven colour yield across a single carpet tile.
Two other, but less commonly used, methods of fixing dyestuff to carpet or carpet tiles use radiant energy and microwave energy, respectively, as a source of heat. The main problem with radiant energy heaters is that it is difficult to generate the amount of energy necessary to raise the temperature quickly in a carpet tile. In addition, using the radiation generators currently available makes it extremely difficult to obtain uniform heating of the surface of the carpet tile, so that uniform dye fixation and colour yield is difficult to achieve.
Microwave heaters can melt the bituminous materials of the backing of some carpet tiles (bituminous materials being ready absorbers of microwave energy). Heating systems utilising focused microwave energy are expensive. They are also difficult to operate to obtain uniform heating over a significant area.
Both the radiant and microwave heating methods also commonly result in the absorption of significant amounts of energy in the backing of the tile, thus giving reduced thermal efficiency and sometimes causing dimensional instability in the backing.